Light emitting diode (LED) based light-emitting devices are today increasingly used for a wide variety of lighting applications. In particular, organic light-emitting devices (OLEDs) are increasingly used for signage lighting, such as advertising information and indication signs.
For commercialization of OLEDs, device efficiency is an important parameter. The total device efficiency is determined by the internal quantum efficiency and the external quantum efficiency (light out coupling). In recent years many techniques have been proposed to improve the internal quantum efficiency, including for example doping of transport layers to increase charge recombination, and the use of triplet-emitter by which almost 100% internal quantum efficiency can be achieved. Also to improve the external quantum efficiency numerous techniques have been proposed, including particular stack designs for reducing external quantum efficiency roll-off of triplet-emitters based on reduced triplet-triplet annealing, and by choosing various layers with refractive indices and thicknesses such as to reduce reflection of the emitted light at the layer interfaces. For example, U.S. Pat. No. 7,269,062 suggests a hetero-structure organic device created by depositing organic material for e.g. a hole transport layer via inkjet or other techniques, and then cross-linking the deposited organic material to render it insoluble. The process can be repeated for sequential deposition of layers. The cross-linked layers allow flexibility in choosing the band gaps and refractive indices of the various materials to control the position of the recombination zone and also the optical confinement of the photons emitted, which can lead to improvement in both internal and external quantum efficiency.
However, a problem with existing devices is that still, a relatively large portion of light remains trapped in the organic layers or the glass substrate due to total internal refection at layers interfaces. Hence the need for improvement of external quantum efficiency in OLEDs remains.